1. Field of the Invention
The present invention has as its object an improved device for connecting blades to a hub in propellers of fans, aircraft and other applications, with variable fitting whether in the stopped position or in motion.
2. Description of the Related Art
In the application for European Serial No. 90810598.4 of the same Ceute applicant, published on Feb. 20, 1991 with publication No. 0 413 663 A2 with Argentine priority of Aug. 14, 1989, a device is claimed in which the connection of each blade to the hub took place by means of a screw placed outside the contour of the blade.
The screw acted as a pivot for the variation of the fitting of each blade, by ensuring the mechanical blocking of the blade itself to the hub, at the desired fitting.
In the Italian patent application Serial No. 91A 003500 in the name of Elica s.r.l., filed on Dec. 30, 1991, there is a blade-hub connection that falls within the above-mentioned previous European patent application, inasmuch as the attachment of the blades to the hub, although realized in a different manner, has the longitudinal axis of the screw located outside the contour of each blade (see for example FIGS. 13 and 14 of that patent application).
Both in the device according to the EPO patent application, and in the Italian patent application just cited, the blade was practically not disturbed structurally at the base by maintaining the full resisting section in the areas of maximum bending moment.
The disadvantage of the "Ceute" device was that it had to have hubs large in diameter since each blade, for at least 50% of the span had to be superimposed on the hub.
This arrangement did not constitute a drawback for fans of large diameter, made up of blades characterized by significant slenderness (by slenderness is meant the ratio: radius of the blade/span of the blade): the large diameter hub serves to reduce the length of the blade and thus the stresses, and to create a shield to prevent the reverse flow in the area of the hub.
Obviously, this effect can be obtained in a much less costly way by a simple sheet metal, aluminum or plastic disc.
On the other hand, with small diameter fans, the "Ceute" device would compromise the performance of the fan, since it necessitated a hub of excessive diameter with the resulting necessity of fairing upstream from the hub.
The "Elica" device, in practice, although it reduces costs with respect to the solution just examined inasmuch as the cost of the hub decreases significantly, presents the drawback, for small fans, of exhibiting the bases of the blades that are altered from the aerodynamic standpoint.
The presence of support 15 in FIG. 14 in the "Elica" device significantly alters the aerodynamic field at the base.
Both the "Ceute" and the "Elica" devices can find their field of application for axial fans characterized by a ratio of radial extension of the blade outside the support/radius of the blade itself close to the unit.
This ratio is used for the reasons stated above and especially when the fans are made to operate at a high tip speed.
In this case for equal yield, few blades are required, in addition to the significant slenderness, with the high tip speed compensating for the small surface of the blades.
In the last 10 to 20 years, on the other hand, in countries with a high population density, industrial and civilian plants are required to observe ever stricter norms relating to noise.
For this reason, with equal diameter and equal yield, due to the tip speed of the propellers, the major cause of noise must be significantly reduced and the surface area of the blades must increase at least by the square of the ratio: ##EQU1## in which 60 and 45 are the tip speeds of the 1970's and 1990's devices in meters/second. Furthermore by increasing the surface area of the blade or to say it more precisely the solidity (understood as the ratio between the surface of the blades and the surface of the fan disc), aerodynamic interference is manifested between one blade and another, and thus axial fans that operate at 45 meters/second in comparison to 60 meters/second in the 1970's devices, exhibit at least twice the surface area of the blade in comparison to that of years past.
Naturally, with equal diameter and yield, in terms of construction, it is more economical to double the span of the blade rather than the number of blades.
Consequently, in the 1990's devices we are dealing with fans characterized by stubby blades (the opposite of "slender") and thus with the capacity to withstand bending moments 8 times greater as compared to the blades of the 1970's.
In fact, by doubling the dimensions, the resisting moment increases with the cube of the dimensions themselves (2).sup.3 =8, if the span of the 1990's blades is twice the span of the 1970's blades.
Furthermore, modest tip speeds intuitively involve reduced loads, thereby supporting the blades with a simple insert is sufficient even if the blade at the base is disturbed by the holes required to connect the blade to the insert itself using screws.
With this invention, the intention is to combine the advantages of the previous patent, while introducing two important variations which make possible an improvement of the aerodynamic yields and at the same time a significant decrease in cost.
The present invention thus has as its object an improved device that completely fulfills all the requirements specified above.